Cutting of plastic sheet

ABSTRACT

An apparatus for slitting foil (14) of thermoplastics material comprises a mover to move the foil (14) through a slitting station (1), the slitting station (1) having a first slitting blade (2) having a sharp front edge (4) located extend transversely through the path of the moving foil (14), and that part (5) of the blade (2) behind the front edge (4) having each side (6), sharped to be non-intrusive into the slit and melted thermoplastics material foil (14), a heater (8,9) to raise and maintain a temperature of the slitting blade (2) to a temperature equal to or above a melting temperature of the thermoplastics material foil (14) selected to be slit by the slitting blade (2), and a cooling blade (3) located behind and aligned with the slitting blade (2) and having two sides each adapted to be aligned with the corresponding side of the slit of thermoplastics material foil (14) and provide by both contact and radiant heating effects, a substantial cooling effect.

CROSS REFERENCE TO RELATED APPLICATION

Applicant under 35 USC 120 and 35 USC 365(c) claims the benefit of thefiling date of International Application PCT/AU88/00409, filed 19October 1988 and naming the United States as one of the designatedstates. The present application is a continuation of said InternationalApplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to effecting cutting or slitting ofthermoplastics sheet or foil.

2. Description of the Related Art

SUMMARY OF THE INVENTION

The problem to which this invention is directed relates to a difficultyassociated with providing a continuous cutting or slitting of a sheet orsheets of thermoplastics material where such sheet or foil is requiredto be separate for whatever purpose.

Such purpose might occur, for instance, where the thermoplasticsmaterial in sheet form is welded to other sheets and is formed in aplurality of adjacent conduits having integrated water resistant outletchannels integrated therein.

There are efficiencies in forming such channels as a multiple number ofchannels in adjacent relationship but it then becomes necessary toseparate by slitting such conduits one from the other.

There are, accordingly, three sheets of the appropriate thermoplasticsmaterial which are welded together and which have to be slit.

It would appear at first sight that all that would be necessary would beto insert a sharp blade at a slitting station and this would besufficient to effect the slitting as the foil is moved through theslitting station.

Unfortunately, the problem is somewhat more difficult.

A first problem is that any sharpened blade very quickly becomes blunt.

Self-evidently, this is solved by replacing the blade or providing amechanism by which the blade is replaced on a regular basis but thereare costs involved to provide such blades on such a continuingreplacement basis and there is furthermore a necessity that themechanism should be reliable to effect such replacement.

There is, however, a further difficulty in the mere use of a blade.

Because the sheet plastics material can be formed by an extrudingprocess in which the polymer chains may only be approximately aligned ina longitudinal direction, it is found that when a blade is caused to cuton a continuing basis through such sheet, there can be a side trackingeffect which can cause substantial misalignment of the slitting locationand also can cause undue strain on other parts of an apparatus causingthe plastic to pass the slitting station.

Both the life of the cutting blade and the tracking effect can beovercome by providing that the blade, while still sharp, is nonethelesscaused to be heated and kept at a temperature so that the cutting effectis combined with a melting effect as the tape engages the blade.

This implicity implies that the blade is made from a thermallyconducting material and is heated to such a temperature that suchelevated temperature can be rapidly transferred to the approachingplastics material.

One further problem arises, however, in that with a melting of theplastics material, it is found that this causes a substantial build upof material which while partially melted or adhering to parts of theblade, nonetheless accumulate immediately subsequent to the blade andcan reach retained quantities that it is necessary, from time to time,to manually clean such materials away from the location.

Apart from the inconvenience from having to constantly monitor suchbuild up, there is the decided difficulty that if it is overlooked atany time such a build up can interfere eventually with the cuttingprocess and can eventually cause a blockage resisting further passage offoil through a slitting station.

According to this invention then, it is proposed that there be locatedadjacent but behind the first slitting blade, a cooling means whereby tocause resolidification of the plastics material very soon after theslitting action.

The result of this action is to cause such a previously melted plasticsmaterial to stay adhered in the close vicinity of the slit edges so thatit is then carried away by the continuing passage of the plasticsmaterial.

Accordingly, by merely causing a cooling effect behind the first heatedslitting blade, the debris can be caused to remain attached to thepassing plastics material and it, therefore, becomes self cleansing.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of this invention it will now be described inrelation to a preferred embodiment which shall now be described with theassistance of drawings in which:

FIG. 1 is a side elevation of a slitting station constructed inaccordance with the embodiment, and,

FIG. 2 is a plan view from a cross section along the lines 2-2 in FIG.1, showing, furthermore, the approaching thermoplastics foil.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring in detail to the drawings, there is shown a slitting station 1which includes a slitting blade 2 and a cooling blade 3.

The slitting blade 2 has a sharpened front end 4 and a rearwardlyextending body part 5 which is defined by parallel sides 6 and 7.

The slitting blade is comprised of steel and is connected between twoelectrodes 8 and 9 by which a substantial electric current is caused toflow through the generally uniform cross sectional area defined by theblade shape as shown and in this manner is caused to be heated to anappropriate heat.

Such a temperature can be varied in accord with the type and grade ofplastic that is being slit but typically for a polythene plasticsmaterial having a melt temperature of approximately 120 degreescentigrade would mean that a temperature on a surface of the blade of150 degrees centigrade is appropriate.

Located immediately behind the slitting blade 2 and separated only by asmall gap 10, there is provided a cooling blade 3 which is made fromcopper and which is kept cool by a plurality of water carrying conduits11 carrying water that is kept to relatively cold temperature so thatthe blade is kept to a temperature of approximately 10 degreescentigrade.

The cooling blade 3 has sides 12 and 13 which are aligned with the sides6 and 7 of the slitting blade 2 so that any material after slitting willtend to maintain some peripheral contact so that by a combination ofradiant loss and conducting loss, the melted plastics will be rapidlycooled and, therefore, caused to maintain adherence to the passing foilmaterial.

It has been found to be of advantage that the foil material 14 which, inthis embodiment is comprised of three welded together sheets ofpolythene plastics material, is supported during the slitting operationon the outer peripheral surface of a drum 15 which includes a slotcorresponding in position to the lower end of the blade 2.

Such an arrangement assists in keeping the thermoplastics foil in afixed position relative to the slitting action of the blade 2.

This then describes the preferred arrangement.

By using heating to effect at least substantial melting and then quicklyresolidifying the melted material, has two very significant advantageousresults.

The first of these is to remove or at least significantly reduce theextent to which an offsetting bias can be otherwise caused by using acutting blade through extruded polythene sheet type materials or otherextruded thermoplastics materials.

Secondly, while the life of the blade 2 and particularly theeffectiveness of a sharp cutting edge is very extensively extended, theprevious disadvantage of debris build up has been avoided by simplyhardening the debris very quickly and then causing this to be kept withthe moving plastic foil and therefore, being effectively disposed of ona continuing basis.

The results, in any event, have disclosed that this has shown itself tobe a very effective technique for the purposes described.

I claim:
 1. Apparatus for slitting a continuously moving sheet ofthermoplastics material comprising a slitting blade at a first location,means to raise and maintain a temperature of the slitting blade to atemperature equal to or above a melting temperature of thethermoplastics material selected to be slit by the slitting blade, andcooling means at a second location downstream of said slitting bladeadapted to effect a rapid cooling of any melted thermoplastics material,said cooling means including a cooling blade located behind the slittingblade and means to effect a cooling of said cooling blade.
 2. Apparatusas in claim 1 in which said slitting blade is stationary.
 3. A method ofeffecting a slitting of a moving foil of a thermoplastics materialcomprising passing the thermoplastics foil through a slitting station inwhich there is a slitting blade having a sharp front edge and which ismaintained at a temperature sufficiently high to cause melting of thethermoplastics material, and then passing the foil past a cooling bladewhich is maintained at a temperature substantially below the meltingtemperature of the thermoplastics material, thereby effecting retentionof the thus melted plastics with the passing thermoplastics material byeffecting a rapid cooling of such melted material after such slitting.4. A method as in the claim 3 further characterised in that thethermoplastics material is polythene, the slitting blade is held to atemperature of approximately 150 degrees centigrade and the cooling iseffected by a cooling blade which is kept to a temperature ofapproximately 10 degrees centigrade.
 5. Apparatus for slitting a movingfoil of thermoplastics material as said foil moves along a predeterminedpath, said apparatus comprising;(a) a slitting station having a slittingblade having a sharp front edge located to extend transversely throughthe path of the moving foil, a body portion including a pair of parallelsides extending rearwardly from said front edge, and heating means forheating said blade to a temperature above the melt temperature of thethermoplastics material to be slit; (b) a cooling blade which isseparated from said slitting blade and located downstream therefrom inthe direction of travel of said moving foil, said cooling blade havingtwo sides which are respectively aligned with the sides of said slittingblade, and cooling means for cooling said cooling blade; and (c) meansfor moving said foil successively through said slitting station and pastsaid cooling blade in the order named.
 6. Apparatus for slitting foil ofthermoplastics material as in claim 5 in which the slitting blade iscomprised of an electrically conducting metal, and the heating meanscomprise electrical heating means.
 7. Apparatus as in claim 5 in whichthe cooling blade is comprised of a material being a thermal conductor,and the cooling means include a supply of cooling fluid passing throughconduit within the thermal conducting material of the said coolingblade.
 8. Apparatus as in the claim 7 in which the cooling fluid iswater.
 9. Apparatus as in claim 7 in which the slitting blade has sidesdownstream of the sharpened front edge which are parallel one to theother, and the cooling blade has sides which are aligned with arespective side of each of the sides of the said slitting blade. 10.Apparatus as in claim 5 in which there is a slitting station and inwhich there it a drum having a peripheral surface on which the foil issupported as it passes through the slitting station, and the slittingblade has an outermost end engaging into a slot within the peripheralsurface of the drum.
 11. Apparatus as in claim 10 in which said drum andsaid slitting station are on opposite sides of the path of saidthermoplastics material.